Atomic layer deposition is known as a method for depositing a monolayer of target material. Atomic layer deposition differs from for example chemical vapor deposition in that atomic layer deposition takes at least two consecutive process steps (i.e. half-cycles). A first one of these self-limited process steps comprises application of a precursor gas on a substrate's surface. A second one of these self-limited process steps comprises reaction of the precursor material in order to form the monolayer of target material. Atomic layer deposition has the advantage of enabling excellent if not ideal layer thickness control. However, atomic layers are essentially thin. As a result, application of atomic layer deposition for depositing layers with a certain thickness larger than about 10 nanometers usually is rather time-consuming, because numerous atomic layers need to be stacked for obtaining such a layer thickness.
In the known roll-to-roll solutions for instance of the type disclosed in WO2103022339 it is not possible to deposit films with a composition gradient in thickness direction, while that would be highly desired in terms of optimizing electrical-, optical and interfacial properties of these films when integrated into a device. For example, it is known that gradient films can be provided by growing multicomponent oxides deposited by spatial atmospheric atomic layer deposition in a prescribed mixing ratio. For example, AlxZn1-xO films can be deposited by using diethyl zinc (DEZ), trimethyl aluminum (TMA) and water as Zn, Al and O precursors, respectively. When the metal precursors (i.e. TMA and DEZ) are co-injected in the deposition region, the Al/(Al+Zn) ratio can be accurately controlled.